The electric powered passenger vehicle has long been considered one of the most attractive alternatives to conventional interal combustion engine driven types from the standpoints of overall efficiency, environmental impact and, most recently, alternative fuel capability. Many commercial enterprises and private individuals, some under the auspices of the federal government, have proposed various approaches to implementing an electrically powered vehicle. To date, there have been virtually no commercially successful vehicles produced on a large scale.
A large number of approaches to the implementation and control of an electric vehicle are evidenced in the patent literature. Most of the approaches fall within one of three general categories of motive power source. These categories are hybrids, D.C. motor drives and induction motor drives.
The first type, the most frequently found in the patent literature, is the hybrid vehicle, comprising a small gasoline fueled internal combustion engine which mechanically drives an electrical generator which, in turn, supplies electrical energy to an A.C. or D.C. motor. With this arrangement, the gasoline engine can operate at a constant speed (at relatively high efficiency) and achieve a substantial fuel saving compared with an engine employing the conventional wide range of operation. A shortcoming of many hybrids is that they are relatively heavy, requiring an electrical generator and motor as well as the gasoline engine. Additionally, the engine requires substantial amounts of a volatile liquid fuel and generates exhaust emissions.
A second approach taken in the developmment of electric vehicles is the use of a bank of batteries which supply electrical energy to a D.C. motor. A variable speed motor drive circuit provides easy and versatile control of the vehicle. The principal advantage of this arrangement is that a D.C. motor control system requires a relatively simple power and control circuit. Unfortunately, this advantage is often more than offset by the relatively large initial cost and maintenance expenses of the motor itself. In addition, D.C. machinery is relatively heavy and bulky, factors which do not lend themselves well to implementation within a light-weight compact vehicle. Finally, D.C. motors inherently require choppers and commutators which create sparks and RF pollution whch can be controlled only at additional expense.
The third, and most attractive approach from the applicants' viewpoint, is an A.C. vehicle employing a battery bank and an induction motor. A.C. motors are relatively light-weight, inexpensive and efficient when compared to D.C. motors. A.C. induction motors, with no brushes or commutation, are more rugged and reliable than their D.C. counterparts and require substantially less maintenance. Related to power-to-weight ratio is the fact that A.C. machines can be driven at substantially greater speeds than D.C. motors. Because A.C. motors do not generate sparks, they can readily be employed in dusty, explosive and high humid atmospheres or high altitudes. Additionally, A.C. motors can be liquid cooled if the application so requires. Although typically superior to D.C. motors in electric vehicle application, A.C. motors often require complex control circuits which are dedicated to associated vehicle drivetrains.
Historically, A.C. motors on adjustable frequency excitation are operated at a constant terminal voltage per unit frequency (V/HZ). Such an arrangement is acceptable when the motor drives a relatively large and constant load. Under those conditions, the motor will operate at a relatively high level of efficiency. However, under light and/or varying load conditions, losses are encountered which can significantly reduce the motor's overall efficiency. When varying load conditions are anticipated, prior practice has been to establish a compromise constant V/HZ ratio which results in the maintenance of a tolerable efficiency level through the entire range of operation.
The relative low cost of electrical energy has, in the past, discouraged innovation in the area of optimal A.C. motor control. However, the recent precipitous increase in energy costs has altered priorities and dramatically increased the cost effectiveness of what heretofor were considered negligible efficiency improvements.
It will be apparent from a reading of the specification that the present invention may be advantageously utilized in many different control applications, both mobile and fixed. However, the invention is especially useful when applied to electric powered passenger commuter vehicles, which characteristically encounter widely varying motor load conditions, and will be described in connection therewith.